An optical device that is being used in an optical communication system is increasingly miniaturized and is increasingly produced at low cost. For example, in a 10 Gbps optical transceiver, the standard of a small-sized and low-power optical transceiver becomes general as represented by X2 and XFP (10 Giga-bit Small Form Factor Pluggable).
Along with that, the miniaturization of an electro-optical converting unit and a photo-electric converting unit that are used in the optical transceiver is also being advanced. Therefore, the miniaturization standard of TOSA (Transmitter Optical Sub Assembly) and ROSA (Receiver Optical Sub Assembly) becomes general as represented by XMD-MSA (Miniature Device Multi Source Agreement).
In the TOSA and ROSA, an optical component and a printed circuit board are generally connected by a flexible substrate. FIG. 16 is a diagram illustrating the configuration of a conventional optical transceiver 10 that includes flexible substrates. As illustrated in FIG. 16, the optical transceiver 10 includes light receptacles 12, and a ROSA 13a, a TOSA 13b, optical component terminals 14, flexible substrates 15, and a printed circuit board 16.
The optical transceiver 10 performs optical coupling by fitting optical connectors 11 into the light receptacles 12. The positional accuracy for fitting the optical connector 11 into the light receptacle 12 should be generally not more than 100 micrometers. Therefore, optical components such as the TOSA 13b or the ROSA 13a are fixedly arranged in the optical transceiver 10 by using the position of the light receptacle 12 as a standard. Alternatively, the TOSA 13b and the ROSA 13a may not be fixed in the optical transceiver 10 in such a manner that the TOSA and ROSA move to positions at which they are matched with connectors to be inserted.
By arranging the TOSA 13b and the ROSA 13a in the optical transceiver 10 by using the position of the light receptacle 12 as a standard, positional misalignment occurs between the optical component terminals 14 and the printed circuit board 16 due to a contour tolerance and a location tolerance of the ROSA 13a, the TOSA 13b, and the printed circuit board 16. In a connection unit of the optical component terminal 14 and the printed circuit board 16, a connection terminal should be shaped and connected to the printed circuit board 16 due to the positional misalignment in such a manner that the connection terminal is matched with a connection pad position of the printed circuit board 16 by lengthening the connection terminal. Therefore, the characteristic of a high-frequency signal may be degraded and a short may occur between a signal line and a power supply. The relaxation of positional misalignment and the maintenance measures of high frequency characteristic can be performed by using the flexible substrate 15 of which the impedance is controlled.
Moreover, a connection unit 17 in which the optical components (the ROSA 13a and the TOSA 13b), the optical component terminals 14, the flexible substrates 15, and the like are connected has various configurations (for example, see Japanese Laid-open Patent Publication No. 2007-158856 and Mitsubishi Electric Corporation, “XMD-MSA-based 10 Gbps modulator integrated semiconductor laser (EA-LD) module”, [Online], [Aug. 21, 2007], <URL:http://www.mitsubishichips.com/Japan/new_pro/no.118/p18—1.html>). FIGS. 17 and 18 are diagrams illustrating the configuration of a conventional connection unit.
As an example, in a connection unit 20 illustrated in FIG. 17, leads 26 are connected to surface patterns (a high-frequency transmission path 22, surface GNDs 23, and DC terminals 25) of a multilayer ceramic substrate 21, and the leads 26 are inserted into through-holes 28 of a flexible substrate 27 to be connected by solder.
As an example, in a connection unit 30 illustrated in FIG. 18, flying leads 37 of a flexible substrate 36 are electrically connected to surface patterns (a high-frequency transmission path 32, surface GNDs 33, and DC terminals 35) of a multilayer ceramic substrate 31 by using bonding.
However, the conventional art has a problem in that high frequency characteristics are degraded due to discontinuous GNDs and high frequency characteristics are degraded in a transmission path and GNDs as explained below. In a cross-sectional view of the high-frequency transmission path 22 of FIG. 17, electric lines of force of between the signal line and the GNDs are thickly generated to an inner-layer (lower-layer) GND 24 as illustrated in FIG. 19. In the connection method illustrated in FIG. 17, only the surface GNDs 23 of the transmission line are connected to the GND layer of the flexible substrate 27. Because the inner-layer (lower-layer) GND 24 that has thick electric lines of force cannot be directly connected to the GND of the flexible substrate 27 but is connected to the GND through via holes 29, a transmission line becomes discontinuous and thus high frequency characteristics are degraded in the case of a high frequency signal.
It is considered that the gap between the high-frequency transmission path 32 and the surface GNDs 33 is reduced as illustrated in FIG. 18 and the transmission line is formed so that electric lines of force between the signal line and the GNDs are thickly generated at the side of the surface GNDs 33 as illustrated in FIG. 20. When the connection method of FIG. 18 is performed, because the connection of the leads 26 as in FIG. 17 is spatially difficult by reducing the gap between the high-frequency transmission path 32 and the surface GNDs 33, the flexible substrate 36 is directly connected to the high-frequency transmission path 32, or is connected to the high-frequency transmission path 32 by using the flying leads 37 as in FIG. 18. In this case, the discontinuity of a signal line caused by the leads is canceled, and thus high frequency characteristics can be secured by connecting only the surface GNDs. However, to satisfy about 40 GHz high frequency characteristics, it is necessary to keep the gap between the high-frequency transmission path 32 and the surface GNDs 33 less than or equal to 100 micrometers. Therefore, the connection between the flexible substrate 36 and the high-frequency transmission path 32 requires a positional accuracy of about several dozen micrometers. Moreover, the connection unit of the flexible substrate cannot secure sufficient connection strength, and thus the flexible substrate or the flying leads may be damaged.